C# 串口读取数据不一致

Question

I need to read data of length of 18 bytes from serial port in .NET 5

My problem is, when I'm reading 18 bytes, then almost everytime it reads 17 bytes into my 18 bytes buffer. So the data looks like

F1-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
DD-0F-08-7C-23-2E-3C-95-11-00-01-00-02-00-0A-00-07-00

but it should look like:

F1-DD-0F-08-7C-23-2E-3C-95-11-00-01-00-02-00-0A-00-07

first 2 bytes is header - to validate start of data sequence - always same

second byte is length of data - 0x0f - we expect 15 data bytes following

The code is basic:

_readPort = new SerialPort("COM8", 115200, Parity.None, 8);
_readPort.Open();
_readPort.DiscardOutBuffer();
_readPort.DiscardInBuffer();

while (_continue == true)
{
    //Thread.Sleep(1000);
    byte[] buffer = new byte[18];

    try
        {
            _readPort.Read(buffer, 0, 18);
            Console.WriteLine(BitConverter.ToString(buffer));
    }
    catch (TimeoutException) {}
}

Console.ReadLine();
_readPort.Close();

It works if I sleep the thread for 1 second, but thats not the solution I want. I think it should load data correctly. If i tried to load data by 2 bytes in buffer, it behaved very similarly. Doesn't matter, if I try it on real device or if on virtual COM ports - result is same. So I guess there is something wrong with code, but I do not know what, since I'm not experienced with serial port communication.

The length of data for now is 18 bytes, but it can differ in the future. I have a second question, which would help me a lot. Is it correct to load all 18 bytes or should I read just 3 and then load dependently on the lengh of data byte? How to synchronize on that header bytes? This would help me a lot.

Thanks for any answer.

edit after help of Frenchy:

Tried this and it feels for me kinda slow when loading data even when not checking header bytes. But the result is not achieved anyway. Data received:

Header: F1-DD-0F Data: 08-7C-23-2E-24-95-11-00-01-00-02-00-0A-00-07
Header: F1-DD-0F Data: 08-7C-23-2E-22-95-11-00-01-00-02-00-0A-00-07
Header: F1-DD-0F Data: 08-7C-23-2E-2A-95-11-00-01-00-02-00-0A-00-07
Throwing invalid header: F1-00-00

Code:

static void Main(string[] args)
        {
            // prepare thread for reading data from read port
            _readPort = new SerialPort("COM8", 115200, Parity.None, 8);
            _readPort.DataReceived += new SerialDataReceivedEventHandler(PortDataReceived);
            _readPort.ReadTimeout += 1000;
            _readPort.Open();

            Console.ReadKey();
            _readPort.Close();
        }

        static void PortDataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            //if (_readPort.IsOpen) return;

            SerialPort port = (SerialPort)sender;

            byte[] headerBuffer = new byte[3];
            port.Read(headerBuffer, 0, 3);

            if (headerBuffer[0] != 0xf1 || headerBuffer[1] != 0xdd)
            {
                Console.WriteLine("Throwing invalid header: " + BitConverter.ToString(headerBuffer));
                return;
            }

            byte[] dataBuffer = new byte[headerBuffer[2]];
            port.Read(dataBuffer, 0, headerBuffer[2]);

            Console.WriteLine("Header: " + BitConverter.ToString(headerBuffer) + " Data: " + BitConverter.ToString(dataBuffer));
        }

Trying to load 3 bytes to check header validity and data length, but after few reads it fails and over and over reads wrong data. :-(

--- edit 2

I've made it work with the following:

static void PortDataReceived(object sender, SerialDataReceivedEventArgs e)
{
    SerialPort port = (SerialPort)sender;

    byte[] headerBuffer = new byte[3];
    port.Read(headerBuffer, 0, 3);

    if (headerBuffer[0] != 0xf1 || headerBuffer[1] != 0xdd)
    {
        Console.WriteLine("Throwing invalid header: " + 
                BitConverter.ToString(headerBuffer));
                port.DiscardInBuffer();
                return;
    }

    byte[] dataBuffer = new byte[headerBuffer[2]];

    for (int i = 0; i < headerBuffer[2]; i++) {
        dataBuffer[i] = (byte)port.ReadByte();
    }

    Console.WriteLine("Header: " + BitConverter.ToString(headerBuffer) + " Data: " + BitConverter.ToString(dataBuffer));
     }

First I read 3 bytes of data. If first and second byte matches the header value, then we take third byte, which identifies length of data. Then read single bytes in foreach with length received from header. If header is not matched, then throw data away.

It's working, but still there might be a better solution, to read data faster and not to throw 4 out of 5 buffer reads. For now it's enough for me to test. Optimization will come later. Thanks frenchy to helping me!

Answer 1

just one idea, instead of looping why not use the event like this: maybe a better result. Qome questions: are you sure to not have the header FF-DD in your datas, either big risk of desynch? try to decrease the speed..

_readPort = new SerialPort("COM8", 115200, Parity.None, 8);
_readPort.Open();
_readPort.DiscardOutBuffer();
_readPort.DiscardInBuffer();
_readPort.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);

private void port_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
    if (!_readPort.IsOpen) return;
    int bytes = _readPort.BytesToRead;
    byte[] buffer = new byte[bytes];
    _readPort.Read(buffer, 0, bytes);
    Console.WriteLine(BitConverter.ToString(buffer));
}